Shorting transformer windings for PI, why is it done?

[JoshuaSWaller79]

The tests are to measure resistance between the windings and ground, so would you not want to apply the test voltage equally across the device?

Shorting the winding to itself (H1-H2, X1-X2) ensures that there is no difference of potential (voltage) between one end of the winding to the other. With no difference of potential this helps prevent any voltage being transformed to the other winding during the initial application of DC (transients). Shorting the other windings helps ensure that if there is any energy transformed there is no voltage across the windings, and they have to be tested anyway.

Hope this helps.

[SecondGen]

See, I've heard this explination before but I don't understand its effects on the test. Since we are talking DC, I imagine that any transformation in the windings would be very, very brief until the winding under test is polarized. Once voltage is sustained, any induction to the other windings would dissipate almost immediately.

All of this happens during initial charging current, which is usually only a few seconds, so whats the point? I imagine the difference in potential cant be more than a couple volts, so how much induction is actually taking place?

The only thing that instantly comes to mind is preventing voltage spikes that could possibly damage the low side insulation, depending on the rated voltage. Is there something else Im not seeing here?

Shorting the winding to itself (H1-H2, X1-X2) ensures that there is no difference of potential (voltage) between one end of the winding to the other. With no difference of potential this helps prevent any voltage being transformed to the other winding during the initial application of DC (transients). Shorting the other windings helps ensure that if there is any energy transformed there is no voltage across the windings, and they have to be tested anyway.

Hope this helps.

[JoshuaSWaller79]

You bring up a valid point about the voltage spikes potentially damaging the insulation.

The transients would be present during the initial engergization of the tester as well as when it is de-energized. Depending on the ratios involved the transformed voltage could be dangerous. It also seems like a safety precaution to me for the personnel testing the equipment.

See, I've heard this explination before but I don't understand its effects on the test. Since we are talking DC, I imagine that any transformation in the windings would be very, very brief until the winding under test is polarized. Once voltage is sustained, any induction to the other windings would dissipate almost immediately.

All of this happens during initial charging current, which is usually only a few seconds, so whats the point? I imagine the difference in potential cant be more than a couple volts, so how much induction is actually taking place?

The only thing that instantly comes to mind is preventing voltage spikes that could possibly damage the low side insulation, depending on the rated voltage. Is there something else Im not seeing here?

[wooddy]

Well, the argument against using jumpers has to do with the interconnections of the windings. Transformers are wired to where one winding jumps to another. If you look at a delta or why configuration, everything is already tied together. So some people have made the argument that you don’t need to use jumpers when testing because they are already electrically continuous.

But Paul gill and Megger and doble are not stupid people. Their procedure specifically spells out to jumper the windings. I just don’t know the exact reason why

its simply to eliminate winding inductance on the insulation being tested, stresses the winding evenly thus eliminating any winding inductance

[test11]

I am running a transformer resistance check, measuring High to Low winding. If I apply 10kV on bushing H1, what is the voltage at H2 and H3? By bonding the bushings together, I know 10kV is applied to the entire H winding. The same goes for bonding the X windings together.

Open circuit means no voltage drop. The entire high side is whatever you put in. You can prove it to yourself by taking a meter across the winding. It will read zero because there is not a load, therefore, no voltage drop.

[Relay1]

Open circuit means no voltage drop. The entire high side is whatever you put in. You can prove it to yourself by taking a meter across the winding. It will read zero because there is not a load, therefore, no voltage drop.

Take a two winding xfmr, 14400/120-240 center tapped. All bushings floating, ground removed. Energize H1 at 10kv and measure X1. Measure X2. Measure X3. Now bond H1-H2 and bond X1-X2-X3. Energize H1 at 10kv and Measure X1. You should see a difference in your readings. It may not be much of a difference and not enough for you to be concerned with but if I'm paying the bill I would want to know the procedures used are the same as the previous tests and will match the future test procedures as well.
If this was an open circuit there would be no current flow, no voltage drop and an infinite amount of resistance. What we are measuring is the current flow between the windings to develop our resistance reading. Since we have current flow from the energized winding would we not also have a voltage drop?

[test11]

Take a two winding xfmr, 14400/120-240 center tapped. All bushings floating, ground removed. Energize H1 at 10kv and measure X1. Measure X2. Measure X3. Now bond H1-H2 and bond X1-X2-X3. Energize H1 at 10kv and Measure X1. You should see a difference in your readings. It may not be much of a difference and not enough for you to be concerned with but if I'm paying the bill I would want to know the procedures used are the same as the previous tests and will match the future test procedures as well.
If this was an open circuit there would be no current flow, no voltage drop and an infinite amount of resistance. What we are measuring is the current flow between the windings to develop our resistance reading. Since we have current flow from the energized winding would we not also have a voltage drop?

The current flow is from the winding as a whole to ground or other winding with an infinite number of parallel paths of varying resistance. Definite voltage drop of 99.999 percent across the open you're reading unless the insulation is exceptionally poor(weak spot or spots) which would read well below any published specification anyway. There is no current flow through the traditional current path so the voltage difference in the energized winding would be zero. You may see micro or milivolt differences which could easily be noise already present.

As for getting different readings, you could not change a thing, retest, and still get a slightly different reading(if it doesn't peg high). The measurement is just so low to begin with(machine usually reading nanoamps). Not to mention if you're in a humid environment you now have a different surface area of bare energized parts leaking from atmosphere to ground.

Bottom line: do what you want. You said it yourself it was negligible.

[Samuel]

Well, the argument against using jumpers has to do with the interconnections of the windings. Transformers are wired to where one winding jumps to another. If you look at a delta or why configuration, everything is already tied together. So some people have made the argument that you don’t need to use jumpers when testing because they are already electrically continuous.

But Paul gill and Megger and doble are not stupid people. Their procedure specifically spells out to jumper the windings. I just don’t know the exact reason why

I expect this is to keep your DC pulse from inducing a proportional voltage pulse on the other winding. Remember DC is only DC at steady state and in the beginning of application it still creates a changing magnetic field.

[TestTim]

See, I've heard this explination before but I don't understand its effects on the test. Since we are talking DC, I imagine that any transformation in the windings would be very, very brief until the winding under test is polarized. Once voltage is sustained, any induction to the other windings would dissipate almost immediately.

All of this happens during initial charging current, which is usually only a few seconds, so whats the point? I imagine the difference in potential cant be more than a couple volts, so how much induction is actually taking place?

The only thing that instantly comes to mind is preventing voltage spikes that could possibly damage the low side insulation, depending on the rated voltage. Is there something else Im not seeing here?

I got stuck at this problem only because some guy in my team wanted to skip shorting windings of transformer. I was happy to the discussion here. I found some useful information a HV testing technique book which was published in 1970. The reason why we consider shorting winding on each side is that the impulse voltage which caused by IR/PI testing will create voltage differece and current between three phases, which might cause potectial damage to insulation or saftey hazards.

[Primepower1]

I don't think its a misunderstanding, I've seen it in a lot of literature. This is the procedure Paul Gill lays out in his book:

Attachment 224

I will post some more if I get a chance to dig through some megger manuals, I may have also seen it in "A stitch in time."

But, this article does not explain why you jumper the poles. I was told it was to equalize the potential at the ends of all bushings. Why you would do that I don't know. It may effect the DAR reading due to current flowing through the winding connections. I have never experimented to look at the differences.